A variety of optical disks have been used in practical applications for recording thereon and reproducing therefrom information by projecting a laser beam, and there have been ever-increasing demands for higher-density recording.
A high density recording can be realized by reducing a diameter w of a beam spot either by reducing a wavelength .lambda. of a semiconductor laser, or increasing a numerical aperture (NA) of a converging objective lens. However, with the current techniques, it is difficult to reduce the wavelength of the laser beam .lambda.. Therefore, attempts have been made to maximize the NA of the objective lens.
However, by increasing the NA of the objective lens, an aberration resulting from the skew of an optical disk with respect to an optical axis of a light beam and a tilt of the disk increases, which lowers a permissible level with respect to the skew of the disk. This causes the problem that deviation or skipping in tracking servo is likely to occur, which results in degrading of recording and reproducing characteristics.
For a CD (Compact Disk: a substrate thickness (1.2 mm)) as an example of recording/reproducing systems available in the market, the NA of the objective lens is 0.45, and for this case, an allowable level of a skew angle .theta. with respect to an ideal horizontal plane of the disk which permits the aberration to be suppressed within a predetermined range is around .+-.0.6 deg. Since the aberration increases at a ratio proportional to the 3rd power of the NA, to obtain still higher NA of the objective lens to meet demands for higher density recording, the allowable level of the skew angle .theta. becomes too small for practical applications.
However, in such optical disks, in the case of increasing the NA of the objective lens, the allowable level of the skew angle .theta. can be ensured by reducing the thickness d of the disk substrate. Namely, by reducing the thickness of the substrate, the aberration caused by the skew or the tilt of the disk can be reduced. As a result, the allowable level of the skew angle .theta. can be ensured, thereby permitting practical applications of the described optical disks.
To meet demands for high density recording, it is desirable to reduce the thickness of the disk substrate to be not more than 1 mm. In recent years, optical disks having a substrate of 0.6 mm thick or 0.8 mm thick have been proposed and already used in practical applications. For example, for DVDs (Digital Video Disks), two disks, each having a thickness of 0.6 mm are laminated to ensure an allowable level of the skew angle .theta..
However, in the case of overwriting by magnetic field modulation in the magneto-optical recording system, as it is required to move a magnetic head in the close proximity to a recording medium, a substrate of a single-layer structure is needed. Such single-layer substrate is advantageous also in view of reproducing costs.
In the case of adopting a single-layer substrate, a problem of tilt may arise due to changes in stress of the film or ambient temperature and moisture. In an event of a tilt of the substrate, a tilt angle is formed with respect to a light beam incident on a disk, thereby generating the aberration. For this reason, it is preferable that the substrate have an ideal flat surface by minimizing the tilt.
In order to suppress the described tilt of the substrate, various techniques have been proposed. For example, Japanese Unexamined Patent Publication No. 67332/1992 (Tokukaihei 4-67332) discloses the technique of reducing the tilt resulting from the internal stress of an optical recording film by adjusting a stress of a composite resin film for use in protecting the substrate and a stress of a composite resin film for use in protecting the optical recording film.
Another technique has been proposed by Japanese Unexamined Patent Publication No. 303769/1993 (Tokukaihei 5-303769) wherein an anti-moisture-permearance film is formed on an opposite side of the recording face of the substrate, and a protecting film is formed on the anti-moisture-permearance film. Therefore, the substrate can be prevented from having moisture permearated only through the light incident side, thereby preventing the tilt of the disk.
According to the described arrangement, a tilt of an optical disk including a substrate of a predetermined thickness can be reduced. However, in the case where a rigidity of the substrate is lowered, and a tilt of the substrate is more likely to occur like the above case of adopting the thin disk substrate, sufficient effects of suppressing the tilt of the disk cannot be ensured.
Here, the effects of the thinner structure of the substrate on the rigidity or the tilt will be explained.
The flexural rigidity D of a uniform flat plane such as a disk substrate can be represented by the following formula (1): ##EQU1##
E: modulus of longitudinal elasticity of the flat plate (Young's modulus),
d: thickness of the flat plate, and
.nu.: Poisson's ratio of the flat plate.
As shown in the above formula (1), as the flexural rigidity D is proportional to the third power of the thickness of the substrate, the flexural rigidity suddenly abruptly decreases as the substrate becomes thinner, and thus a warpage of the substrate is more likely to occur.
The recording medium film (hereinafter referred to as an optical recording film) of the optical disk can be fabricated, for example, by vaporization, sputtering, etc. However, the internal stress remains in the film itself by the film forming process, resulting in the tilt of the disk. The described problem becomes obvious for optical disks having small flexural rigidity of the thin structure like the described arrangement. Further, such tilt angle becomes the skew angle when recording and reproducing, and thus an attempt of increasing the allowable level of the tilt angle .theta. fails as the tilt of the substrate itself becomes high and the tilt angle .theta. may even exceed the allowable level.
Further, there exists a difference in mechanical physical values between the optical recording film and the disk substrate thus fabricated. Namely, due to the difference in the thermal expansion, a uniform flat plate such as the disk substrate, etc., may be subjected to flexure by the bimetallic effects. In this case, provided that the strain of the optical recording film is equivalent to the strain of the disk substrate at an interface between them, the film thickness d.sub.f of the optical recording film is significantly smaller than the thickness d.sub.s of the substrate, and the radial tilt angle .phi. of the optical recording film is very small, then the radial tilt angle .phi. at the radius position r from the center of the disk can be represented by the following formula (2). ##EQU2##
E: Modulus of longitudinal elasticity (Young's modulus),
.nu.: Poisson's ratio,
d: Thickness of the optical recording film or the disk substrate,
.alpha.: Linear thermal expansion coefficient, and
.DELTA.T: Changes in temperature.
Here, the subscripts f and s indicate the optical recording film and the disk substrate respectively. For the tilt angle .phi., a positive direction is defined to be a direction in which the substrate is extended.
According to the formula (2), in the case of adopting the optical recording film having different coefficient of linear thermal expansion and the modulus of longitudinal elasticity from those of the disk substrate is formed on the disk substrate, bimetallic effects are induced by the difference in the thermal expansion between the two layers proportional to temperature changes, resulting in the tilt of the optical disk.
For optical disks, it is required to have practically operable temperature range between some minus .degree. C. and +60.degree. C. Therefore, in this example, the temperature range is set between 25.degree. C. and 55.degree. C. (.DELTA.T=30.degree. C.), and an optical disk including a polycarbonate disk substrate with a diameter of 120 mm, and a thickness of 0 6 nm, and an optical recording film made of Fe with a thickness of 200 nm is used. By converting the numerical values for the above conditions into physical values, the radial tilt angle .phi. has a physical value of 1.6 deg. Provided that the above disk has a completely flat surface with a tilt angle of 0.degree. at 25.degree. C., then, the tilt angle has changed from the initial value against temperature changes of .+-.30.degree. C. by .+-.1.6 deg. This result is by far larger outside the operable range between .+-.0.4 and 0.6 deg as regulated under conventional CDs, DVDs, magneto-optical disks. It is therefore considered that the above disk cannot be used in practically operable temperature range.
The following example will be given through a rewritable magneto-optical disk in reference to FIG. 6 through FIG. 8. FIG. 6 and FIG. 7 show cross-sectional view of the magneto-optical disk. FIG. 8 shows a change in radial tilt angle under normal conditions in response to temperature change. The magneto-optical disk shown in FIG. 6 and FIG. 7 includes a polycarbonate substrate 101 with a diameter of 120 mm and a thickness of 0.6 mm having formed on its recording surface of a magneto-optical recording film 102 with a thickness of around 200 nm and a protective film 103 made of ultraviolet ray setting resin laminated in this order.
In response to an increase in temperature of the magneto-optical disk, the bimetallic effects are induced, and the substrate 101 warps in its extending direction as shown in FIG. 7. This is because a coefficient of expansion of the substrate 101 is larger than a coefficient of expansion of the optical recording film 102. As a result, as shown in FIG. 8, a change in radial tilt on the average of 0.8 deg. is absorbed in response to temperature changes between 25.degree. C. and 55.degree. C.
The problem of changes in tilt against temperature changes or the initial tilt generated by the internal stress when forming become more noticeable as the disk is made thinner to be suited for high density recording. Namely, to attain high density recording, it is necessary to suppress the initial tilt or changes in tilt by some means.
The techniques of suppressing tilt generated by the internal stress of each film and the moisture disclosed by Japanese Unexamined Patent Publication No 67332/1992 (Tokukaihei 4-67332) and Japanese Unexamined Patent Publication No. 303769/1993 (Tokukaihei 5-303769) are known. However, the techniques disclosed by these publications are not be applied to disk substrates of thin structure in which the flexibility against warpage is low and the bimetallic effects cannot be ignored.
Japanese Unexamined Patent Publication No. 67332/1992 (Tokukaihei 4-67332) which adopts a composite resin film as the stress adjusting means is not suited for reducing a large tilt angle, and fails to provide a solution to suppress a large tilt angle. Namely, Japanese Unexamined Patent Publication No 67332/1992 fails to provide a solution to improve changes in radial tilt against temperature changes which becomes noticeable for the substrates of thin structure.
Japanese Unexamined Patent Publication No. 303769/1993 (Tokukaihei 5-303769) adopts the anti-moisture-permearance film of a thin structure. Therefore, it fails to provide a solution to the described problems. Namely, this publication fails to provide a solution of suppressing changes in radial tilt against temperature changes.